If you do want to stay inland at first, as Pol says the only one with a "full on" registration is Lomond, however be aware there are HP restrictions on other lochs, where you will probably be OK based on what I guess you have on your transom, but above certain HP it can get expensive to lauhch or banned. (E.G. Earn - <50Hp it;s something stioopid like £3, >50 it's £15 kind of thing. Some of the small fishing Lochs above Callander are limited to 2 or 3 HP....)
Google does shed a lot of light.
Lochs Tay, Awe and the Caley Canal also provide lots of open, fresh water.